1. Field of the Invention
The present invention relates generally to the field of communication systems and more particularly to the field of cellular telephone communication between communication systems having different signaling protocols.
2. Description of Background Art
In recent years the use of cellular telephones and cellular communications systems has significantly increased. Many of the early communication systems were based upon analog technology, e.g., the advanced mobile phone system (AMPS). In recent years the protocols used by some of these analog systems have been modified to add features, e.g., call forwarding and authentication, and some digital systems have been developed that use digital signaling protocols, e.g., the digital communication system (DCS1900). FIG. 1 is an illustration of a conventional cellular communication network. The communication network includes a mobile station (MS) 102, e.g., a dual-mode AMPS/DCS1900 cellular telephone that can operate using an AMPS protocol and the DCS1900 protocol which will be manufactured and commercially available from Motorola Corporation. The MS 102 transmits and receives an electromagnetic signal to a base station (BS) 104 via the air. In FIG. 1, four BSs 104 are illustrated. Two BS 104A-B are part of a first system, e.g., an IS-41B/AMPS system, and two BSs 104C-D are part of a second system, e.g., a DCS1900 system. Typically, a BS 104 is a tower having an antenna and a transceiver. The MS 102 communicates with a BS 104 whose physical location is near the MS 102. Currently, there are thousands of BSs 104 in the United States. Some of the functions performed by each BS 104B include converting the received electromagnetic signal into an electrical signal, assigning a voice channel to the MS 102, and paging an MS 102. The BS 104 transmits a signal representing a unique MS identifier to a mobile switching center (MSC) using a signaling protocol supported by the MSC. The IS-41B signaling protocol is described in greater detail in the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA), Interim Standard IS-41-B, Electronic Industries Association, December 1991 that is incorporated by reference herein in its entirety. The IS-41C signaling protocol is described in greater detail in the TIA/EIA, Interim Standard IS-41-C, Cellular Radio-Telecommunications Intersystem operations, February 1996 that is incorporated by reference herein in its entirety. Frequently, the MSC includes a visitor location register (VLR) and together are illustrated in FIG. 1 as an MSC/VLR 106. Currently, there are hundreds of MSC/VLRs 106 in the United States. Frequently, in the AMPS system, each MSC/VLR 106A-B and its associated BSs 104A-B are provided by a single manufacturer, e.g., Motorola Corp. Accordingly, a proprietary interface protocol is typically used to communicate between the BSs 104A-B and the MSC 106A-B. For DCS1900 systems, the DCS1900 standard protocol is used to communicate between the BSs 104C-D and the MSC 106C. The DCS1900 protocol is described in greater detail in TIA/EIA Interim Stantdard IS-652, February 1996 which is incorporated by reference herein in its entirety.
The MSC/VLR 106A identifies the home system, e.g., system 2, of the MS 102 and transmits a signal through a signaling network 108, e.g., the signaling system 7 (SS7) which is a standard telecommunication signaling system, to the home system for the purpose of registering the MS 102. The home system includes an MSC/VLR 106C and a home location register (HLR) 110B having a profile of the subscriber (registered user) of the MS 102. A subscriber profile includes a copy of the unique MS identifier and indications of the type of services available to the subscriber, e.g., call forwarding and multi-party calling. One purpose of registering the MS 102 with the home system is to enable the MSC/VLR 106A to charge the home system so that the home system can charge the owner of the MS 104 for the cost of the call on the cellular network. If the unique MS identifier matches a stored identifier in the home system, then the home system transmits a registration signal to the MSC/VLR 106A and the user of the MS 102 is then permitted to use the cellular network.
One problem with conventional systems is that if the protocol utilized by the first system to which the MS 102 is transmitting is incompatible with the protocol utilized by the second system which is the home system of the MS 102, then registration signals transmitted by the MSC/VLR 106A in the first system will not be properly interpreted by the HLR 110 of the second system. A conventional solution to this problem is to have a dual-mode HLR in the home system that is capable of interpreting the protocols utilized in both the first system and the second system. In the present example, the dual-mode HLR would be capable of transmitting and receiving signals using the IS-41B protocol and the DCS1900 protocol. A problem with this solution is that developing and manufacturing a dual-mode HLR is expensive and is only capable of interpreting signals received by the HLR itself. That is, each HLR must have this dual-mode capability.
Another problem with conventional systems occurs when the first system illustrated in FIG. 1 supports a protocol, e.g., IS-41C, that is compatible with the protocol supported by the second system, e.g., IS-41B, but the first system offers more features to the user, e.g., authentication. In conventional systems, the MS 102 can not take advantage of the additional features offered by the first system when traveling through the area supported the first system. For example, a significant problem with current cellular networks is fraud. Specifically, the signal between the MS 102 and the BS 104A can be intercepted by an unauthorized source. The unauthorized source can determine the unique MS identifier included in the intercepted signal and can then program a fraudulent MS to transmit the intercepted MS identifier. As a result, the unauthorized source utilizes the cellular network and the cost of this use is charged to the subscriber whose MS identifier was intercepted. Some estimates of the monetary losses from this type of fraud range from $2 million to $3 million per day, as of 1996. Fraud typically occurs more frequently in larger cities. Accordingly, many system providers in the larger cities have upgraded the signaling protocol in order to include an authentication feature. For example, the IS-41C signaling protocol supports authentication while the IS-41A and IS-41B signaling protocols do not support authentication. Conventional systems do not permit a dual-mode MS 102 to utilize the authentication capabilities (or other additional features) of the IS-41C signaling protocol while traveling through an area supported by the first system if the home system does not support authentication (or other additional features), e.g., if the home system only supports the IS-41B signaling protocol.
The authentication standards developed by the TIA, for example the TSB51, are described in TIA/EIA, Telecommunications System Bulletin--TSB51, Cellular Radiotelecommunications: Authentication, Signaling Message Encryption and Voice Privacy, May 1993, that is incorporated by reference herein in its entirety, and the TIA IS-41-C. The TIA air interface standards are: IS-54-B that is described in TIA/EIA, Interim Standard IS-54-B, Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard, April 1992; IS-88, described in TIA/EIA, Interim Standard IS-88, Mobile Station--Land Station Compatibility Standard for Dual-Mode Narrowband Analog Cellular Technology, January, 1993; IS-91, described in TIA/EIA Interim Standard IS-91, Mobile Station--Base Station Compatibility Standard for 800 MHz Analog Cellular, October, 1994; IS-95-A, described in TIA/EIA, Interim Standard IS-95-A, Mobile Station--Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems, May, 1995; and IS-136, described in TIA/EIA, Interim Standard IS-136, 800 MHz TDMA Cellular--Radio Interface--Mobile Station--Base Station Compatibility, December, 1994, that are all incorporated by reference herein in their entirety. In addition, a more detailed description of the SS7 network is described in the American National Standards Institute, Inc. (ANSI), American National Standard for Telecommunications, Signaling System Number 7 (SS7)--General Information; Exchange Carriers Standards Association Committee T1; T1.111-1992 which is incorporated by reference herein in its entirety.
What is needed is a system and method for (1) enabling two or more communication systems each supporting a different and possible incompatible signaling protocol to communicate with each other where the system and method are not located within any of the affected systems; and (2) enabling a mobile station to utilize features supported by the visited system even if its home system does not support the feature.